feat: accept match alternatives in an ensures clause - #14701
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This PR lets the `requires` clause of a `def` contract be written like a `fun`, so a precondition may destructure the arguments of the assertion: `requires | (a, b) => a = 0 ∧ b = n` matches on the state of a state monad. The term form `requires P` and the binder form `requires s => P s` are unchanged. The clause parser gains `Term.matchAlts` as an alternative, and `expandDefContract` wraps it in the `fun` that becomes the precondition of the specification theorem.
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Moves the fun-like clause body from `requires`, whose arguments are the assertion's own and carry no shape to match on, to `ensures`, which binds the result. Both clauses now start on their own line when pretty printed, so a clause can no longer trail the alternatives of the one before it.
sgraf812
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requires clauseensures clause
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August 6, 2026 08:43
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Benchmark results for 559fd19 against c4e6b62 are in. No significant results found. @sgraf812
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This PR lets the
ensuresclause of adefcontract be written like afun, so a postcondition may be stated per shape of the result:ensures | none => False | some v => 2 * v ≤ n. A contract clause now also starts on its own line when pretty printed, as it is written in source.ensuresClausegainsTerm.matchAltsas an alternative andexpandDefContractwraps it in thefunthat becomes the postcondition of the specification theorem. I decided to keeprequiresas is, because matching on state arguments of the assertion is not a use case I intend to support.